ICTJA PhD presentation Award 2019 - Núria Bach (Poster Presentation)

First approximation to the petrography and mineral chemistry  of the Maladeta and Andorra-Mont-Louis plutons (SIMPROP  Project).

Introduction: what is SIMPROP?
Simprop is a coordinated project with the main goal of studying the relationship between the plutonic and volcanic processes of siliceous magmas as part of the same magmatic system, with special emphasis on their temporal, structural, petrologic and geodynamic relationships, based on the multidisciplinary study of the Permo-Carboniferous magmatism of the Catalan Pyrenees. SIMPROP is divided in two sub-projects: one will be carried out by CSIC researchers that will deal with the study of the magmatic products. The other will address the characterization of structure from a geophysical perspective and will be carried out by IGME.

Geological setting
The Maladeta and the Andorra-Montlouis plutons constitute two of the main variscan granitic bodies outcropping in the Pyrenean Axial Zone (Fig.1). These bodies were intruded mainly during the last stage of the variscan orogeny (Late Carboniferous-Early Permian) between 310 Ma and 290 Ma (between 310 Ma and 290 Ma (Lopez-Sanchez et al., 2018; Esteban et al., 2015; Denèle et al., 2014; Druguet et al., 2014; Maurel et al., 2004; Evans et al., 1998; Romer and Soler, 1995) and uplifted afterwards during the Alpine orogeny

Figure 1. Geological map of the area. a) situation of the different variscan bodies outcropping in the pyrenees. Modified from french Geoportail (https://www. geoportail. gouv.fr/), Institut Cartogràfic i Geològic de Catalunya (ICGC) and Porquet et al., 2017. b) detailed map of the two studied bodies with location of the samples studied until date. Modified from Pereira et al., 2014. 

The first remarkable difference between the volcanic sample studied (S-1) and the granitic ones (S-2, S-21, S-40, S-49, S-50, S-55, S-58) is the absence/presence of hornblende (Fig. 2). Whereas in volcanic rocks hornblende forms phenocrysts up to 1 mm, this mineral has not been observed until date in plutonic rocks. Some references of this mineral in the granites are found in the bibliography (Maurel et al., 2004), however in low concentrations. 

Figure 2: comparison between a volcanic (a, b) and a plutonic sample (c, d) from two different thin sections of the Andorra – Mont-Louis pluton in transmitted microscopy in parallel light (a,c) and polarized light (b,d). Plag: plagioclase, Kf: K-feldspar, Qtz: quartz, Hbl: hornblende, Bt: biotite.

Plutonic samples present an equigranular texture with quartz, plagioclase, K-feldspar and biotite as main minerals (Fig. 3a-g). Muscovite has only been found until date in some samples from the Maladeta pluton (Fig. 3h).

Figure 3: different features of the granitic samples under transmitted microscopy in polarized light. a-d) plagioclase with polysynthetic twining, zoned and often altered to sericite. K-feldspar presents perthites (e,f) and can be observed interstitially filling fractures along quartz crystals (g) . h) muscovite in the Maladeta sample i) prehnite with some reaction and dissolved borders appears cutting biotite.

Biotite often presents zircon and apatite inclusions (Fig. 4). Its remarkable to observe the abundance of ilmenite inclusions in the inner part of the Andorra- Mont-Louis pluton (Fig. 4b,c) whereas its presence is rare and small in the external part of the pluton and in the Maladeta massif (Fig. 4a). 
Figure 4: different inclusions in biotite grains under EDS. It is remarkable the difference of ilmenite inclusions between the center and the rim of the Andorra – Mont-Louis massif. Ap: apatite, ilm: ilmenite, Zr: zircon

Mineral geochemistry
Regarding the mineral chemistry, plagioclase ranges from albite to labradorite being oligoclase and andesite the most frequent composition (Fig. 5). No relevant difference is observed between the different granitic facies and between the two different plutons. 

Figure 5: composition of different feldspar grains from three different samples analyzed using microprobe.

Future work

• Determine the connection between the volcanic and the plutonic products (petrography, mineral and major geochemistry).
• Crustal or mantelic source? Determine the genesis of magmas: mineralogy (cordierite and muscovite vs. hornblende and biotite), mineral geochemistry, Nd/Sr isotopes.
• Emplacement mechanisms of the magma: one magmatic pulse vs. different pulses? (diffusive modelling, geophysical methods: magnetotellurics, magnetic susceptibility, AMS, density and gravimetric studies .
• Dating the different events and gaps à Schamuells, S.
• Crystallization sequence of the granites and dyke emplacement. When did they intrude?

Denèle, Y., Laumonier, B., Paquette, J. L., Olivier, P., Gleizes, G., & Barbey, P. (2014). Timing of granite emplacement, crustal flow and gneiss dome formation in the Variscan segment of the Pyrenees. Geological Society Special Publication, 405(1), 265–287. https://doi.org/10.1144/SP405.5
Druguet, E., Castro, A., Chichorro, M., Pereira, M. F., & Fernández, C. (2014). Zircon geochronology of intrusive rocks from Cap de Creus, Eastern Pyrenees. Geological Magazine, 151(6), 1095–1114. https://doi.org/10.1017/S0016756814000041
Esteban, J. J., Aranguren, A., Cuevas, J., Hilario, A., Tubiá, J. M., Larionov, A., & Sergeev, S. (2015). Is there a time lag between the metamorphism and emplacement of plutons in the Axial Zone of the Pyrenees? Geological Magazine, 152(5), 935–941. https://doi.org/10.1017/S001675681500014X
Evans, N. G., Gleizes, G., Leblanc, D., & Bouchez, J. L. (1998). Syntectonic emplacement of the Maladeta granite (Pyrenees) deduced from relationships between Hercynian deformation and contact metamorphism. Journal of the Geological Society, 155(1), 209–216. https://doi.org/10.1144/gsjgs.155.1.0209
Gisbert, J. (1981). Estudio geológico–petrológico del Estefaniense-Pérmico de la Sierra del Cadí (Pirineo de Lérida): Diagénesis y sedimentología. Ph.D. Thesis, University of Zaragoza
Lopez-Sanchez, M. A., García-Sansegundo, J., & Martínez, F. J. (2019). The significance of early Permian and early Carboniferous U–Pb zircon ages in the Bossòst and Lys-Caillaouas granitoids (Pyrenean Axial Zone). Geological Journal, 54(4), 2048–2063. https://doi.org/10.1002/gj.3283.
Martí, J. (1986). El volcanisme explosiu tardihercinia del Pirineu Català. Ph.D. Thesis, University of Barcelona.
Martí, J. (1991). Caldera-like structures related to Permo-Carboniferous volcanism of the Catalan Pyrenees (NE Spain). Journal of Volcanology and Geothermal Research, 45(3–4), 173–186. https://doi.org/10.1016/0377-0273(91)90057-7
Maurel, O., Respaut, J. P., Monié, P., Arnaud, N., & Brunel, M. (2004). Ages U-Pb de mise en place et 40Ar/39Ar de refroidissement de la partie orientale du pluton granitique de Mont-Louis (Pyrénées orientales, France). Comptes Rendus - Geoscience, 336(12), 1091–1098. https://doi.org/10.1016/j.crte.2004.04.005
Pereira, M. F., Castro, A., Chichorro, M., Fernández, C., Díaz-Alvarado, J., Martí, J., & Rodríguez, C. (2014). Chronological link between deep-seated processes in magma chambers and eruptions: Permo-Carboniferous magmatism in the core of Pangaea (Southern Pyrenees). Gondwana Research, 25(1), 290–308. https://doi.org/10.1016/j.gr.2013.03.009.
Porquet, M., Pueyo, E. L., Román-Berdiel, T., Olivier, P., Longares, L. A., Cuevas, J., … Vegas, N. (2017). Anisotropy of magnetic susceptibility of the Pyrenean granites. Journal of Maps, 13(2), 438–448. https://doi.org/10.1080/17445647.2017.1302364
Romer, R. L., & Soler, A. (1995). U-Pb age and lead isotopic characterization of Au-bearing skarn related to the Andorra granite (central Pyrenees, Spain). Mineralium Deposita, 30(5), 374–383. https://doi.org/10.1007/BF00202280
This research is supported financiall by SIMPROP- CGL2017-84901-C2-1-P. Thanks to Joan Marti (principal ID and PhD supervisor

ICTJA PhD Presentation Award 2019 - Olaya Dorado García (Poster Presentation)


Volcanoes are among the most amazing and yet dramatic natural phenomena. Their beauty and, more than anything, their natural richness have captivated the humans since historical times. But the same forces that allow this natural development are responsible of one of the most catastrophic events in planet earth: the volcanic eruptions. 

Tenerife island is a highly populated and touristic área. Teide and Pico Viejo stratovolcanoes are located in the center of the island but their return periods are way much longer that the mafic eruptions located in the dorsal vents. That’s why historically they haven’t been considered as a potential risk on the island. But, the truth is that they are active volcanoes and that they are capable of much violent and explosive phonolitic eruptions. A better knowledge of this type of eruptions and the main factors that controls these changes in eruptive styles are required to undertake a comprehensive volcanic hazard assessment of Tenerife island. In order to achieve that, we need to study the geology of the volcanic deposits to understand how the volcanic system have worked in the past.

Geological map and volcanostratigraphy from Martí et al. (2011).

Phonolitic eruptions in Tenerife can occur in two different vents: central vents (ex. Teide volcano) and numerous lateral dome vents (around T-PV stratovolcanoes). Generally, eruptions originated in central vents tend to be essentially effusive while dome eruptions present a much more explosive behaviour. This explosive dynamic is typical of the beginning of the eruption and then is followed by an effusive phase, with the emission of lava flows. But, which are the driving forces of the abrupt transitions between explosive and effusive activity during an eruption?

Reality is very complicated: there are a lot of factors that can interplay between each other to determine eruptive style. Different studies have suggested, for example, compositional and/or volatiles zonation in the magma chamber, magma ascent rate, degassing processes, influx of new magma into the chamber, changes in the pre-eruptive conditions, etc. As we need to simplify this, we chose to focus this study in the pre-eruptive conditions, that is, the temperature, pressure and volatile content (basically water content) within the differents parts of the magma chamber related to each eruptive phase.

With that in mind, we have conducted a petrological and mineral characterization of the different eruption phases of Pico Cabras dome eruption with the objective of identifying the pre-eruptive parameters that control these changes in the volcanic activity.

For that, we sample rocks from the explosive (pumices) and effusive (lava flows) phases. Using both petrographic and scanning electron microscopes we can identify the minerals and glass that form that rocks. Also, we can know their chemical composition by analysing them with an electron microprobe. With that data (chemical composition of clinopyroxenes and their magma in equilibrium) we can use a geothermobarometer to obtain the temperature and the pressure at which the minerals were form. Also, using a geohygrometers (that uses the composition of the feldspars and the magma in equilibrium) we can obtain how much water was dissolved in the magma. However, the reality is much more complicated because it is really, really difficult, to find the exact composition of the minerals parental magma. For that, we had to calibrate our results comparing them with experimental petrology data.

Once the rocks, the minerals and the chemical composition have gave us all the information about each eruptive phase, we can start to understand how the magma chamber was and what were the processes occurring prior and during the eruption. Our results suggest the presence of a compositionally stratified magma chamber at 1 kbar±0.5kbar in which the differences in the eruptive styles are controlled by the temperature and the amount of volatiles dissolved in the melt. The explosive phase is related to the upper part of the magma chamber at 725ºC±25ºC and 3,5-5 wt% H2O and the effusive phase with the main body of the chamber at 880ºC±30ºC and 2,5-3 wt% H2O.

Feldspar zonation also show us that the minerals travelled inside different parts of the magma chamber with slightly different compositions in a process defined in the literature as self-mixing. They also suggest that the eruption was triggered by an injection of mafic magma right beneath the magma chamber (underplating). This injection doesn’t imply magma mixing, but only the contribution of temperature could have increased the energy inside the magma chamber and trigger the eruption.

Finally, we have also found evidences of halogen volátiles (principally Cl and Br) in the pumice samples originated from the explosive phase: we have identified a sodalite crystal, a Cl-rich mineral that hadn’t been found yet in recent T-PV magmas. The release of this kind of volatiles into the atmosphere have a direct impact on the ozone layer depletion and it's other factor that should be taken into account in future risk studies.

Further reading and references in: http://hdl.handle.net/2445/141802